1. Field of the Invention
The present invention relates to an image forming apparatus employing electrophotography-processes, electrostatic recording processes, or the like, which can be preferably applied to a color copying machine, a color laser printer, and so on.
2. Related Background Art
Various kinds of color image forming apparatus employing electrophotography processes, electrostatic recording processes, or the like have been devised and put to practical use. FIG. 9 shows an example of the color laser printer which repeats a rotation transfer process several times to obtain a multicolor image.
The color laser printer of this embodiment has a photosensitive drum (image bearing member) 1 on which a latent image is formed and which can rotate in the direction indicated by the arrow in the FIG. 9. Around the periphery of the photosensitive drum 1, a primary charger device 2, a laser exposure device 3 for forming the latent image on the photosensitive drum 1, a developer device 4 having a plurality of developers 4Y, 4M, 4C and 4B for converting the latent image formed on the photosensitive drum 1 into a visible image (toner image), and a transfer drum 5 for transferring the toner image on the photosensitive drum 1 onto a transfer medium P are arranged.
The transfer medium (sheet) is separated from a sheet feed unit one by one by a sheet feed roller 6, and sent to the transfer drum 5 by regist rollers 7 at constant timing. The transfer drum has a nipping means 14 such as a gripper for nipping the front end of the transfer medium P fed to the transfer drum 5. The transfer medium P is electrostatically attracted onto the surface of the transfer drum 5 by an attracting roller 13 to be further conveyed. The transfer medium P supported and conveyed by the transfer drum 5 passes between the transfer drum 5 and the photosensitive drum 1 (transfer position), where the toner image formed on the photosensitive drum 1 is transferred. After this transfer process is repeated several times, the transfer medium P is separated from the transfer drum 5 by a separation claw 15 and conveyed to a fixing device 8 by a convey belt 16. The fixing device 8 fixes each color toner image into a permanent image to obtain a multicolor image.
In the above-mentioned conventional apparatus, when a transparent resin sheet such as an OHP sheet is used as the transfer medium P, the OHP sheet separated from the transfer drum 5 is also conveyed to the fixing device 8 by the convey belt 16. In order to ensure melting of toner on the transfer medium and prevent fixing failure, however, fixing takes about twice as long time as a normal sheet of about 100 g. For, heat capacity of a normal sheet and that of an OHP sheet much differ from each other, and the OHP sheet needs several times as much quantity of heat as the normal sheet of the same thickness to obtain practical transparency.
In order to solve this problem, fixing speed of the fixing device 8 must be made less than 1/2 of the peripheral speed of the transfer drum 5 when the OHP sheet is conveyed. Accordingly, conveying speed of the convey belt 16 is set to be substantially equal to the peripheral speed of the transfer drum 5 until the rear end of the OHP sheet separates from the transfer position, but after separation, the conveying speed is decelerated to be less than 1/2 of the peripheral speed of the transfer drum 5.
On the other hand, when the transfer medium P is a normal sheet of about 100 g, the peripheral speed of the transfer drum 5 and the conveying speed of the convey belt 16 are set to be substantially equal to the speed at which the transfer medium passes through the fixing device.
Thus, the convey belt 16 effectively adjusts the transfer medium conveying speed according to the time required for fixing different kinds of transfer media P. However, it is necessary to make the transfer sheet conveying path between the transfer position and the fixing position longer than the maximum length of the transfer sheet employed. Therefore, the convey belt 16 must be rather long, which is problematic in miniaturizing the apparatus main body.